Various types of transport and reversal apparatus have been proposed, in order to transport the sheet from one printing station to another and, during the transport, reversing the face of the sheet so that the sheet is turned over in its path between succeeding printing stations. One such apparatus--described, for example, in German Pat. No. 12 62 294--utilizes printing stations having plate and/or rubber blanket cylinders of a predetermined diameter, and an impression or printing cylinder of twice the diameter. The impression or printing cylinder is formed with grippers to grip the leading sheets being passed between the printing cylinder or the rubber blanket cylinder and the impression cylinder. The transport and reversal cylinder, likewise, has twice the diameter of the plate and/or rubber blanket cylinder.
A transfer and reversal drum or cylinder is located between the impression cylinders of the respective printing stations. The apparatus functions in this manner: For reversal of a sheet gripped by the grippers of the impression cylinders, the transfer cylinder is spaced slightly from the impression cylinder and, as the sheet passes along the transfer cylinder, the trailing end portion of the sheet is gripped by the reversal and transport cylinder. Gripping, usually, is carried out by suction grippers, since placement of mechanical grippers to catch the trailing edge or end portion of a sheet is difficult if sheets of various lengths have to be handled. The reversing and transport drum or cylinder then transports the sheet to the second printing station, the reversal cylinder gripping the previously trailing edge of the sheet which, now, forms the leading edge of the sheet as it is applied to the second printing station.
The sheets may have different lengths, and it is therefore not possible to utilize the mechanical gripper system on the reversal and transport cylinder which can accomodate various lengths of sheets. This is due to the construction of mechanical grippers which customarily grip behind the edge of the sheet to be engaged. It would only be possible to accomodate various lengths of sheets if, in the impression cylinder, grooves were to be cut to match the gripper position to respective sheets. This, however, cannot be done for printing reasons, since stripes might form on the printed subject matter.
To reverse and transport a sheet, therefore, it has been proposed to utilize suction grippers which can engage the trailing zone of a sheet, rather than being placed to accurately grip an edge thereof. German Pat. No. 12 62 294 describes such a suction gripper arrangement. The suction grippers engage the sheet in the rear or trailing end region or zone, lift off the end region or zone and, by a simple pivoting movement, pivot the rear sheet edge or zone into the interior of the transfer cylinder, thereby lifting the trailing end portion or zone of the sheet off the prior impression cylinder. The end zone of the sheet can then be transferred to a customary mechanical gripper, since the cylinder has rotated in the meanwhile, the grippers then no longer being in danger of collision with the impression cylinder.
The system works satisfactorily with thin substrate sheets; when handling heavy or stiff substrates, however, such as cardboard, "Bristol board" or the like, substantial force must be applied on the suction grippers. The sheet is stressed, and the forces transferred to the sheet must accomodate the sheet, since the sheet first must be braked with respect to its original direction of movement, and then the direction of movement reversed. Even slight slippage between the suction grippers and the end zone of the sheet, thus, may interfere with proper register and decrease the quality of printing in the subsequent printing station with respect to the first printing station.